udp广播问题？

我写了个简单的程序，用的是idudpClient和idudpServer,其中,发送广播并接收部分写进了一个线程内,但是,当先打开客户端时,有错误;先打开服务器时，正常。其中，线程内代码如下：unit SendThread;interface
  uses Classes,SysUtils,Controls,Windows,IdComponent,IdUDPClient,ComCtrls,MainForm;
type  TDelectThread=class(TThread)
   private
      IdUDPC:TIdUDPClient;
      LV:TListView;
   protected
       procedure  ExeCute;override;
   public
        constructor  Create(IdUDPC1:TIdUDPClient;LV1:TListView);
        destructor   Destroy;override;
end;
implementation{ TDelectThread }constructor TDelectThread.Create(IdUDPC1: TIdUDPClient; LV1: TListView);
begin
   IdUDPC:=IdUDPC1;
   LV:=LV1;
   inherited Create(False);
end;destructor TDelectThread.Destroy;
begin
  inherited Destroy;
end;procedure TDelectThread.ExeCute;
var ReceiveString,RemoteIP:String;
    j:Integer;
begin
   //inherited;
   FreeOnTerminate:=True;
   if WaitForSingleObject(hMutex,INFINITE)=WAIT_OBJECT_0 then
   begin
       if IdUDPC.Active<>True then Exit;
       IdUDPC.Send('GetIp');//此时客户机上如果收到此信息，就把ip地址传过来
       RemoteIP:=IdUDPC.ReceiveString();
       sum:=i;
       try
         if RemoteIP<>'' then
            begin
               for j:=0 to sum-1 do   //????????
               begin
*************************************************
                  if RemoteIP=LV.Items[j].Caption then  Exit   //如果先运行客户端，最后再运行服务器端，此处出错****************************************
                  else
                     continue;
               end;
               LV.Items.Add;
               LV.Items[i].Caption:=RemoteIP;
               Inc(i);
            end;
       except
         IdUDPC.Active:=False;
       end;
   end;
   ReleaseMutex(hMutex);
end;end.
错误出在
        if RemoteIP=LV.Items[j].Caption  这句。请指教？